1. Field of the Invention
The present invention relates to a semiconductor optical amplifier (SOA) that amplifies and outputs incident light and, more specifically, to a method for manufacturing a semiconductor optical amplifier (SOA) having a planar buried heterostructure (PBH) in which a spot size converter (SSC) with a double-core structure is integrated.
2. Discussion of Related Art
Generally, an semiconductor optical amplifier that amplifies and outputs incident light intensity has an input unit and an output unit each having a symmetric structure, and for the semiconductor optical amplifier integrated with a spot size converter, there can be a butt joint semiconductor optical amplifier and a double-core structure semiconductor optical amplifier.
The butt joint semiconductor amplifier consists of an active waveguide region for signal gain and a passive waveguide region for converting an optical mode, in which the active waveguide and a vertical taper type passive waveguide are coupled horizontally.
The double-core structure semiconductor optical amplifier consists of an upper active waveguide and an underlying passive waveguide, in which the upper active waveguide is formed in a horizontal taper type where the width is gradually reduced, so that the efficient light coupling is derived between the underlying passive waveguide and the upper active waveguide.
FIGS. 1A and 1B illustrate a conventional semiconductor optical amplifier fabricated with the butt joint and the selective area growth (SAG).
As a planar buried heterostructure (PHB) where an active waveguide 11 and a passive waveguide 14 are formed on a substrate 10, the active waveguide 11 where an active layer 13 is formed between separated confinement heterostructure (SCH) 12 and the passive waveguide 14 where a vertical taper area 16 is formed are coupled by the butt joint.
The feature of this structure is that the passive waveguide 14 is stacked by a selective area growth (SAG) method to be a vertical taper structure, wherein the farther it is from the active waveguide 11, the thinner the passive waveguide is, whereas the bigger the size of the optical mode becomes, and with such vertical taper structure, the light coupling efficiency with the optical fiber is increased. The vertical taper structure passive waveguide 14 is connected to the active waveguide 11, which is for signal amplification, through the horizontal butt joint 15, and a current blocking layer 17 is formed near the passive waveguide 14 and the active waveguide 11 to improve the current injection characteristics.
However, in the above structure, an inner reflection at the butt joint and a light coupling loss between two waveguides can be generated according to the coupling surface structure and the regrowth condition due to the butt joint between two waveguides having different refractive indices, and a thickness and a composition of the waveguide layer is changed according to the position of a mask pattern when the passive waveguide 14 is grown using the selective area growth (SAG) method. Therefore, the regrowth condition affects the characteristics of the semiconductor optical amplifier significantly, so that a strict growth condition is required.
FIGS. 2A and 2B show a conventional semiconductor optical amplifier with a vertical double-core structure consisting of a lower waveguide and an upper waveguide in a horizontal taper form.
The above semiconductor optical amplifier has a vertical double-core structure, wherein a passive waveguide 22 and an active waveguide 23 are formed, comprising the upper active waveguide 23 for signal gain; the underlying passive waveguide 22 for amplifying the light coupling efficiency with the optical fiber; and a horizontal taper area 24 for deriving the efficient light coupling between the upper active waveguide 23 and the underlying passive waveguide 22 by gradually reducing the width of the upper active waveguide 23. In FIGS. 2A and 2B, the reference numeral 21 indicates a p-InP cladding layer, numeral 27 indicates an ohmic layer, numeral 28 indicates a dielectric layer and numeral 29 indicates an electrode.
The feature of the above structure is that the width of the end of the upper active waveguide 23 is reduced, thereby light propagating through the upper active waveguide 23 is transferred to the underlying passive waveguide 22 without any loss, and that the underlying passive waveguide 22 is thin and its refractive index is small, thereby increasing the size of the optical mode to reduce the light coupling efficiency with the optical fiber aligned at both ends. When the angle of the horizontal taper area 24 is sufficiently small and the width of the taper end is small, the optical coupling loss between the upper active waveguide 23 and the underlying passive waveguide 22 is small and the composition and the structure of the underlying passive waveguide 22 is optimized, thereby enhancing the light coupling efficiency with the optical fiber.
For blocking the current, in the above structure, the upper active waveguide 23 and the underlying passive waveguide 22 are etched, and then the upper cladding layer 21 is stacked and proton ions with high energy are implanted to form the current block layer 25. When the current blocking layer 25 is formed by the proton injection, as described above, the upper active waveguide 23 and the underlying passive waveguide 22 are vertically formed with an InP space layer therebetween, so that the high energy ion is injected into the upper active waveguide 23 and the underlying passive waveguide 22 region to incur the waveguide loss. Therefore, in order to prevent the waveguide loss, a mask pattern capable of blocking the proton injection should be used. However, in case the width of the mask pattern is larger than that of the active waveguide, the efficient current blocking is not achieved, so that there exists a difficulty in reducing the width of the mask pattern as possible. Further, a stress can be generated due to the growth difference at each plane in growing the thick p-InP cladding layer 21, caused by unnecessary planes formed during each etching process for forming the upper active waveguide 23 and the underlying passive waveguide 22, and when the stress is beyond the threshold value, there occurs a problem, such as dislocation. Due to the dislocation and the inefficient current blocking, the leakage current is generated so that the characteristics of the optical device can be degraded and the light loss in the optical mode can be derived at the interface between the active waveguide 23 and the passive waveguide 22 and the p-InP cladding layer 21.